Cables for the transportation of energy are generally composed of at least one central electrical conductor, surrounded by an intermediate electrical insulator, itself protected from the external environment by an external protective sheath. The conductor is generally composed of metal wires, of aluminum or of copper, assembled into strands. The external sheath is normally made of an electrically insulating thermoplastic material. In the past, the intermediate electrical insulator was formed from a thermoplastic material. More recently, the thermoplastic material has been replaced by crosslinked materials, essentially in order to bring the operating temperature of the cables to 90° C., with the possibility of an emergency overload temperature above 100° C. This has made it possible to increase the transportation capacity of power grids, a capacity limited by the heating of the conductors by Joule effect which depends on the properties of the metals used to form said conductors.
When the cable is intended to be subjected to a voltage greater than approximately one thousand volts, the cable additionally comprises conductive layers which are extruded or taped. The role of these conductor layers is to evenly distribute the electric field at the interfaces of the intermediate electrical insulators in order to prevent point effects and consequently to limit the risks of electrical breakdown.
Thus, the cables of this type generally comprise a first internal semiconducting shield in contact with the conductor and a second external semiconducting shield in contact with a metal shield which is intended to collect the leakage currents or the short circuit current, in the case of an incident, and which is itself in contact with the external sheath. The metal shield is connected to earth via a protection system which will trigger the opening of the circuit if the intensity which passes through the shield becomes too high. In such constructions, the complex formed of the intermediate electrical insulator surrounded by the two semiconducting shields is known as a trilayer.
In order to obtain crosslinked trilayers, it is normal in cable manufacture to use intermediate electrical insulators and/or semiconducting shields, the material of which is composed mainly of low density polyethylene to which additives, in particular peroxides, are added. These peroxides, the decomposition of which will result in the formation of radicals, will make it possible to create, in the polyethylene, a three-dimensional network which will provide the thermal stability and which will prevent mechanical deformation of the cable at the operating temperatures, that is to say 90° C. to 110° C.
It is known to choose the peroxide so that its rate of decomposition is virtually zero at ambient temperature, low at the temperature of conversion by extrusion of the material intended to form the intermediate electrical insulator and/or the semiconducting shields, and maximum at higher temperature.
In order to bring about such a rise in temperature, the material intended to form the intermediate electrical insulator is introduced, at ambient temperature, in the form of granules, into the hopper of an extruder and then melted in the screw of the extruder between 120 and 140° C. in general, in order to be brought to a plastic state and a viscosity which is sufficiently low to be shaped around the conductor. It is the same for the two semiconducting shields, which are generally coextruded, with the result that there is found, at the outlet of the triple-extrusion head of the extruder, a complex which still has to be crosslinked.
This operation takes place in a pipe of a few tens of, indeed even a few hundred, meters which is directly connected to the extrusion head via a telescopic part.
In the first part of the pipe, the cable is heated so as to make possible the decomposition of the peroxide or peroxides used and thus the crosslinking of the polyethylene. This heating can be obtained with a heat-exchange fluid, such as steam or oil. It is also possible for the complex to be heated by radiation of the pipe in a neutral atmosphere, such as gaseous nitrogen. In order to render more uniform the temperature of the components to be crosslinked (intermediate electrical insulator and semiconducting shields), recourse has also been had to systems which will heat the central conductor, for example by induction. The temperatures encountered in this first part of the pipe can be between 200 and 400° C., as a function of the nature of the materials involved, of the heating process used, of the geometry of the cable and of the manufacturing rate.
Following the first heating part, the pipe comprises a second part which is devoted to the cooling of the complex. In general, this cooling is obtained by passing the cable into cold water, which circulates continuously in the second part of the pipe by virtue of pumps, so as to maintain a relatively low constant temperature. On the more sophisticated lines and for the cables intended to be subjected to the highest voltages, the cooling can also be obtained by passing through an atmosphere of neutral gas, generally nitrogen, which circulates continuously in the second part of the pipe.
At the outlet of the pipe, the cable obtained has a completely crosslinked insulating trilayer and is at a sufficiently low temperature to be able to be wound onto a receiving cable drum without the cable being permanently deformed by the cable drum.
“Long-die” installations in which the crosslinking is obtained directly in the extruder are also known.
However, in all cases, during the heating of the material intended to form the intermediate electrical insulator and the semiconducting shields, the peroxides decompose to form the radicals necessary for the crosslinking of the polyethylene. In point of fact, the decomposition of the peroxides brings about the formation of by-products which are in fact molecules having lower molecular weights than those of the radicals and which are found trapped at the core of the three-dimensional network created in the polyethylene by the radicals of the peroxides. The formation of these by-products is particularly significant in the case of material intended to form the intermediate electrical insulator.
A portion of the by-products are gaseous at the crosslinking temperatures encountered in the first part of the pipe. It is in order to prevent the formation of bubbles in the intermediate electrical insulator and the semiconducting shields that the first part of the pipe is maintained under pressure between 8 and 25 bar. This is because such bubbles are particularly harmful to the electrical quality of the intermediate electrical insulator and the semiconducting shields.
After passing through the second part of the pipe, these by-products are still present in the dissolved state in the intermediate electrical insulator and the semiconducting shields, the crystallinity of the material of these preventing the formation of bubbles.
Thus, when the cable has exited from the pipe and been placed on the cable drum, it is still not ready for the following manufacturing operations and in particular for the positioning of the metal shield and the extrusion of the external sheath.
This is because a portion of the by-products, usually the most volatile portion, diffuses through the intermediate electrical insulator and the semiconducting shields and escapes toward the atmosphere. For example, for dicumyl peroxide, which is widely used in cable manufacture, the volatile by-products are methane and water vapor. If the manufacture of the cable is continued immediately after exiting from the pipe, for example by positioning the metal shield along the cable, the gas which is given off from the trilayer migrates to the ends of the metal shield and brings about the expansion of the metal shield. Once the cable is in use, this can bring about an electrical incident or alternatively an explosion.
For this reason, it is known not to immediately continue the manufacture of the cable when said cable has exited from the pipe. For a given period known as degassing stage, the cable which has exited from the pipe is left wound on a cable drum, which makes it possible for the most volatile by-products from the freshly manufactured trilayer to be discharged.
For the medium-voltage cables, this degassing stage generally takes a few days in general and is carried out at ambient temperature. Above an intermediate electrical insulator thickness of 6 millimeters, it is, however, necessary to condition the cables for approximately ten days at a temperature of between 50 and 90° C.
Thus, the degassing stage proves to be particularly long and considerably slows down the process for the manufacture of the cables, in particular of the cables dedicated to the transportation of high-voltage electrical energy, the thickness of the intermediate electrical insulator of which is greater. In addition, the degassing stage requires having available a large space for storing the cables. Furthermore, for the case of the very thick cables, the degassing stage proves to be energy intensive and requires having available appropriate ovens.
Solutions for reducing the content of volatile by-products produced during the stage of crosslinking the cable are known from the documents EP 1 944 327 and WO 2012/010640. The degassing stage is thus shortened, which makes it possible to accelerate the process for the manufacture of the cable in the end.